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Recent results from an ITPA joint experiment to study the onset, growth, and
decay of relativistic electrons (REs) indicate that energy loss mechanisms
other than collisional damping may play a dominant role in the dynamics of
the RE population. Understanding the physics of RE growth and mitigation is
motivated by the theoretical prediction that disruptions of full-current
ITER discharges could generate $\sim $10 MA of REs (10-20 MeV) through an
avalanche growth process [M. Rosenbluth, S. Putvinski, Nucl Fusion 37 (1997)
1355]. A necessary condition for avalanche growth is that the Coulomb
acceleration due to the toroidal electric field has to be at least high
enough to counter the collisional drag on background electrons, i.e.
E\textgreater E$_{\mathrm{c}}$, where E$_{\mathrm{c}}$ is the critical
E-field derived in [J. Connor, R. Hastie, Nucl Fusion 15 (1975) 415].
E$_{\mathrm{c}}$ scales linearly with electron density, n$_{\mathrm{e}}$, so
one way to suppress avalanche growth is to quickly raise n$_{\mathrm{e}}$
sufficiently high, but this is problematic on ITER. However, if there are
other energy loss mechanisms in addition to collisions, then the actual
threshold E-field will be greater than E$_{\mathrm{c}}$, i.e. REs become
more difficult to generate and sustain due to the additional loss
mechanism(s). Due to the importance of E$_{\mathrm{c}}$ to the issue of REs
in ITER, the ITPA MHD group is conducting a joint experiment to measure the
threshold E-field on a number of tokamaks under steady-state, low
Z$_{\mathrm{eff}}$ conditions in which V$_{\mathrm{loop}}$,
n$_{\mathrm{e}}$, and REs can be well-diagnosed, and compared to theory. The
analysis must take into account the RE growth time, which can be comparable
to the discharge timescale. Data from DIII-D, C-Mod, TEXTOR, and FTU have
been obtained so far, and the consensus to date is that the threshold
E-field is significantly higher than E$_{\mathrm{c}}$, or conversely, the
n$_{\mathrm{e}}$ required to damp REs is significantly less than predicted,
suggesting that other loss mechanisms are involved. Implications for RE
mitigation in ITER will be discussed.

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2013.DPP.BI2.3